feat(sdk): HubClient.submit() with JSON inputs and outputs

Wrap the Submit RPC so callers can run a flow inline with typed
inputs and read its outputs. jsonInputs (each value a Dart Map) are
converted to a google.protobuf.Struct via a recursive builder and
sent as Payload.json — the only path that can feed a json-typed
module input (e.g. econ.skm_score, law.benefit_score). The returned
SubmitResponse.outputs carry the flow outputs, which the audit log
does not persist.

Signed-off-by: flemming-it <stefan.a.flemming@googlemail.com>

lib/src/hub_client.dart

@@ -4,6 +4,7 @@
 // public method maps onto exactly one RPC on Hub or HubAdmin, so
 // callers don't have to know which service hosts which call.
 
+import 'dart:convert';
 import 'dart:typed_data';
 
 import 'package:fixnum/fixnum.dart';
@@ -17,6 +18,8 @@ import 'generated/chain/v1/hub.pb.dart' as pb;
 import 'generated/chain/v1/hub.pbgrpc.dart' as grpc_hub;
 import 'package:protobuf/well_known_types/google/protobuf/empty.pb.dart'
     show Empty;
+import 'package:protobuf/well_known_types/google/protobuf/struct.pb.dart'
+    show Struct, Value, ListValue, NullValue;
 
 /// Connection coordinates for a hub endpoint.
 class HubEndpoint {
@@ -752,12 +755,87 @@ class HubClient {
     return _admin.runSavedFlow(req);
   }
 
+  /// Submit a flow YAML to the hub for a one-shot run, with typed
+  /// inputs, and read its outputs from the response.
+  ///
+  /// Unlike [runSavedFlow] (which runs a flow already saved in the
+  /// hub by name), this sends the flow definition inline and exposes
+  /// the full [Submit] RPC. Two things only this path can do:
+  ///   * pass **JSON-typed inputs** ([jsonInputs]) — for modules that
+  ///     read `inputs.require_json(...)` (e.g. `econ.skm_score`,
+  ///     `law.benefit_score`). Each value must be a JSON object (a
+  ///     Dart `Map`); it is converted to a `google.protobuf.Struct`.
+  ///   * read the flow's **outputs** off the response. The audit log
+  ///     does not persist outputs — they come back only here.
+  ///
+  /// [textInputs] → text Payloads, [fileInputs] → bytes Payloads
+  /// (MIME from [fileMimeTypes], default `application/octet-stream`).
+  /// All maps default to empty.
+  Future<SubmitResponse> submit({
+    required String flowYaml,
+    Map<String, String> textInputs = const {},
+    Map<String, Uint8List> fileInputs = const {},
+    Map<String, Object?> jsonInputs = const {},
+    Map<String, String> fileMimeTypes = const {},
+  }) async {
+    final req = pb.SubmitRequest()..flowYaml = utf8.encode(flowYaml);
+    for (final entry in textInputs.entries) {
+      req.inputs[entry.key] = pb_common.Payload()..text = entry.value;
+    }
+    for (final entry in fileInputs.entries) {
+      final mime = fileMimeTypes[entry.key] ?? 'application/octet-stream';
+      req.inputs[entry.key] = pb_common.Payload()
+        ..bytes = (pb_common.Bytes()
+          ..mimeType = mime
+          ..data = entry.value);
+    }
+    for (final entry in jsonInputs.entries) {
+      final value = entry.value;
+      if (value is! Map) {
+        throw ArgumentError(
+          'jsonInputs["${entry.key}"] must be a JSON object (Map); '
+          'got ${value.runtimeType}',
+        );
+      }
+      req.inputs[entry.key] = pb_common.Payload()..json = _structFromMap(value);
+    }
+    return _hub.submit(req);
+  }
+
   /// Closes the gRPC channel. Idempotent.
   Future<void> close() async {
     await _channel.shutdown();
   }
 }
 
+/// Convert a Dart JSON object into a `google.protobuf.Struct`.
+/// Mirrors the proto3-JSON mapping the hub expects for a
+/// `Payload.json` input. Keys are stringified; values recurse via
+/// [_structValue].
+Struct _structFromMap(Map<dynamic, dynamic> map) {
+  final s = Struct();
+  map.forEach((k, v) => s.fields[k.toString()] = _structValue(v));
+  return s;
+}
+
+/// Convert a single Dart JSON value into a `google.protobuf.Value`.
+Value _structValue(Object? v) {
+  if (v == null) return Value()..nullValue = NullValue.NULL_VALUE;
+  if (v is bool) return Value()..boolValue = v;
+  if (v is num) return Value()..numberValue = v.toDouble();
+  if (v is String) return Value()..stringValue = v;
+  if (v is Map) return Value()..structValue = _structFromMap(v);
+  if (v is Iterable) {
+    final list = ListValue();
+    for (final e in v) {
+      list.values.add(_structValue(e));
+    }
+    return Value()..listValue = list;
+  }
+  // Unknown type — stringify so one stray value can't fail the run.
+  return Value()..stringValue = v.toString();
+}
+
 /// One declared input in a saved flow's `inputs:` section.
 /// Returned by [HubClient.getFlowDefinition] so GUI clients
 /// can build a typed run-flow form. The [type] field carries